Video recording and reproducing apparatus, particularly apparatus that record and reproduce video information in conformance with broadcast standards are extremely complex, containing sophisticated electronic and mechanical components and systems. They require an extraordinary level of sophistication to provide the amount of control necessary to operate reliably in conformance with broadcast quality standards. Such apparatus accordingly have a number of servo systems to control their operation, including a servo system that controls the rotation of the rotatable video recording and reproducing transducers or heads, a servo system that controls the motors for driving the tape supply and take-up reels, a servo system that controls the correction of the timing of the reproduced video signal, usually called a time base corrector, and a capstan servo system that controls the speed at which the tape is transported. The servo systems often operate under microprocessor control.
Recently developed recording and reproducing apparatus include a head tracking position servo system which controls a movable element carrying the video reproducing head (or heads) mounted on a rotatable drum that also serves as a guide for the tape as it is transported along a helical path around the rotatable head during record and reproduce operations. The movable element is controlled to have the reproducing head follow a recorded video track in a manner permitting reproduction of broadcast quality special effects as when the tape is transported at a speed other than the normal speed at which it was transported during recording of the tracks of video information. During such operations, the head tracking position servo moves the head transversely with respect to the direction of the recorded video tracks so that the head follows each recorded track accurately and produces a quality signal, whether the tape is transported faster than normal record speed, resulting in a fast motion effect, or slower than normal record speed, resulting in a slow motion effect, or even when the transport of the tape is stopped, resulting in a stop motion (still frame or field) effect.
One very desirable feature that has not been available in prior art video tape recording and reproducing apparatus is the capability of providing asynchronous reproduction so that precise time expansion or compression of a segment of recorded video information, sometimes referred to herein as a program, can be reproduced within a shorter or longer interval than the real time interval of the segment. Time expansion or compression is achieved by altering the speed of the transport of the tape from the normal record and reproduce speed and occasionally repeating or skipping the reproduction of a recorded track to effect such expansion or compression.
When the tape speed is so altered, it is highly desirable to have the recording and reproducing apparatus servo locked to the system timing reference in order to provide accurate control of the playback time expansion and compression. Such servo locking is also highly desirable, if not essential, when a recorded video information that is compressed or expanded in time is reproduced by a video recording and reproducing apparatus other than that which recorded the information. With such servo locking, physical variances between transport mechanisms lead to undesirable time variations in the reproduced video information.
Also, if the tape is being transported at a speed other than the normal record and reproduce speed, a slight degree of mistracking occurs. For program expansion or compression, an occasional repeat or skip of the reproduction of a recorded field of video is required to avoid disturbances in the display of the information reproduced from the tape. Thus a head tracking position servo system is virtually a requirement for providing broadcast quality reproduction of the recorded video information under such circum- stances.
Systems are known that provide asynchronous playback of recorded video information to time compress or expand a segment of the video information without the video record and reproduce apparatus being servo locked to the system timing reference. In such systems, the amount of time compression or expansion is specified as a percentage of program figure that represents the desired percentage of time by which the program length is to be varied. Such systems are undesirable for altering the program length of broadcast video information, because the failure to servo lock the reproduction of the recorded video information to the system timing reference usually results in the alteration of the selected time expansion or compression of the program. The alteration is caused by the commonly occurring perturbations in the operation of video record and reproduce apparatus that introduce timing errors into the reproduced video information. Also, the known systems require computations by the operator to determine the correct percentage of program figure. The necessity of such computations allows the possibility of errors that result in erroneous altered program length, which is particularly unacceptable in broadcast applications where precise broadcast schedules must be maintained.
Accordingly, it is an object of the present invention to provide an improved servo system for the control of the transport of tape in a recording and reproducing apparatus, which reliably accomplishes accurate and effective asynchronous playback at tape transport speeds other than normal playback speed. Another object of the present invention is to provide an improved video tape transport control servo system suitable for asynchronous playback which permits an operator to specify the desired time compression or expansion of segments of video information precisely in time units without the necessity of computations. Yet another object of the present invention is to provide an improved video tape transport control servo system that eliminates interchange timing variations, i.e., timing variations resulting from the reproduction of video information by a video recording and reproducing apparatus other than the apparatus that recorded the video information.
Other objects and advantages will become apparent from the following detailed description accompanied by the attached drawings.